In gasoline service stations, there is normally a submersible pump for pumping gasoline from an underground storage tank to a gasoline-dispensing pump. This submersible pump is typically located in a manhole. The manhole is normally formed by a metal cylindrical container known as a manhole skirt having a substantially flat bottom and a top suitable for receiving a manhole cover. The submersible pump is located at the bottom of the manhole. To detect the presence of leaking gasoline which will collect within the manhole, a sensor is provided. It is desirable to make this sensor relatively sensitive to detect small amounts of gasoline to avoid accumulation of large amounts of gasoline and the obvious fire hazard. Unfortunately, the typical sensor cannot tell the difference between gasoline and other liquids such as water. Therefore, if the seal between the manhole and the manhole cover is not watertight, water can leak into the manhole and set off the sensor, which results in shutting down of the submersible pump. This is a problem not only if it rains, but also if a hose is used to wash down the pavement at the service station. When the submersible pump shuts down, customers will not be able to pump gasoline and will take their business elsewhere. Accordingly, to avoid the loss of customers as well as the inconvenience of having to reset the system or the hazard of deactivating the sensor, it would be desirable to prevent water from entering the manhole. Moreover, any water which gets into the system will be contaminated with gasoline and must be disposed of as a hazardous material, which is expensive. Furthermore, if water gets into the system and freezes, the expansion of the frozen water can damage pipes, the pump, sensor, or other components.
Entry of water into manholes of all types, such as for vapor recovery systems and fill pipes for gasoline storage tanks, telephone equipment and electrical equipment can be prevented with the invention. Water can damage such equipment, corrode it, and make it cumbersome and dangerous to work on such equipment.